The Marconi R1155 is a World War II era receiver produced for the British Royal Air Force. It was used in Lancaster and Halifax bombers and includes special circuitry for direction finding.

The R1155 tunes longwave down to 75KHz and shortwave up to 18MHz, with a couple of gaps along the way. Fortunately the broadcast band is covered from 600KHz to 1500KHz, so there is still something to tune in today. Aside from the DF circuitry it's a fairly standard multi-band super-het with one RF stage, mixer-oscillator, two IF stages, detector, one audio stage, and a magic-eye tuning indicator.

An audio power output stage was not included as the receiver was only used with headphones originally. There is no internal power supply as B+ and filament power were supplied from an external source when installed in an aircraft. The unit below the receiver in the photo is a home-made power supply and audio amplifier, described further below.

Many of these receivers were sold as surplus after the war. In Canada they were sold by the War Assets Corporation (see advertisement: XTAL, Jan 1947). They were commonly purchased by hams. Many of them were then extensively modified, typically by removing the DF circuitry and/or installing a power supply or audio amplifier inside. This unit came through the surplus/ham lineage but fortunately remained in essentially original condition. None of the circuitry had been removed or modified, all the removable shields were still present and the rubber-insulated wiring was still in quite good condition. The only modification that had been made prior to my receiving it in the mid-1990's was the addition of an octal connector for a power supply. I removed this once some large-pin Jones plugs for the original connectors were found. Repairs amounted to replacing one leaky 0.1 uF bypass capacitor!

One minor modification I made internally was to bypass the low end of the volume control with a larger-value capacitor, as the original circuit does not permit the volume to be adjusted down to 0. A dial illumination lamp has also been added as there was none originally.

A schematic from reverse-engineering is available. The original schematic is available on the web but this one is a little clearer regarding the switching arrangements.

Home-made Power Supply & Audio Amplifier

This is a home-made power supply and audio amplifier for the R1155. The case was constructed in a manner similar to that of the receiver and was designed to integrate in appearance with the R1155.

Mechanical

The enclosure:

• The corner braces are 1-inch angle bracket (1/8 inch thick) (6 pieces).
• The side, top and bottom panels are simply chassis-gauge sheet aluminum (4 pieces). The top panel folds down to form the back as well. The only trick here is in forming the bull nose on the front edges. Rather than simply folding the edge over, fold it over something to give some thickness to the nose, so forming it into a beam to provide some rigidity along it's length. This can be done in a 3-step process:
  1. Form the edge into a 90 degree angle.
  2. Butt another piece of material (1/16-inch thick or so) having a straight edge into the right angle and secure. Fold the edge over this material to 180 degrees.
  3. Remove the extra piece and press the back of the edge down to the main surface (fold it a little beyond 180 degrees).

The front panel and chassis:

• The front panel is a 19-inch-rack panel (1/8-inch thick) cut down to size.
• Note the brackets attaching the chassis to the front panel have edge flanges folded into them for rigidity.
• The U-brace at the back is present simply to provide a stable rest when the chassis is turned upside-down to work on.

It occurred to me while doing this that if one had a junker R1155, the enclosure could be used with a new front panel and chassis.

Electronics

The power supply requirements of the R1155 are a little unusual in that the B- is not ground. Consequently, a floating B supply must be supplied to the receiver: the receiver uses the B- for grid bias (C-) and derives the ground reference internally.

The transformer was scavenged from an old, discarded tube TV. An adjustable power resistor was included in the B+ filter string for adjusting the B+ voltage. A 6V6 was used in a simple audio output stage.

It is far simpler of course to use solid-state elements here. In this instance, I went the tube route for the sake of 'period consistency', although this principle is violated with the modern IEC power connector. If one wants to be really pedantic the 6V6 was not around either when the R1155 was produced.

Improvements

A couple of things I would probably do differently if I were doing it again:

• The small-pin Jones connector on the power supply chassis seems to be noisy, a better connector type may be preferable.
• Try using the headphone transformer output (on connector N1) from the receiver to feed the audio amp, rather than the plate connection (on connector N2). This would keep all the connections on the same connector, avoiding the solitary connection to N2.
• Arrange the front panel to include or permit installation of a DF indicator.

External Links

• Some specs & c.
• Bo Samuelsson's restoration, includes DF indicator.
• Peter Holtham's guide to rebuilding the R1155.
• Parts for sale from G4CVU.
• VMARS tech data for the R1155. (2012 Jul)
• GMOUHC's blog of R1155 rebuild. Links on pg64. (2012 Jul)